Although the application to ink jets of a cell according to the invention is not limitative, it is this application which will be described in greater detail in the following description. We find in fact an illustration of the main functions which such a cell may accomplish taken separately or in combination with another cell. It should be recalled here what are the particular requirements which a circuit must comply with for supplying a continuous ink jet printing head with ink. It is a question more particularly:
of generating an ink jet the flow rate is generally less than 20 cm.sup.3 /min and this at a pressure which may reach 4 bars;
of residual fluctuations in the supply pressure of less than 1%;
of recovering and recycling the whole of the ink flow generated that is not used for printing;
of the possibility of using inks with very volatile solvents making rapid drying possible on nonporous materials--such as metal or glass, for example;
of high reliability; and
of completely automatic operation in an industrial environment, without maintenance and without a demanding cleaning procedure before prolonged shut down of the supply circuit.
In the ink jet marking printers known at present, different solutions have been chosen for complying with the above expressed requirements. For example, gear pumps are used providing the functions of pressurizing the jet and depressurizing the gutter for recover of the jet, for cooperating with integrated means for measuring viscosity, and for adding solvents when the ink used includes volatile solvents. A supply circuit of this type is described in French patent application 8316 440 filed by the applicant and published under the number 2 553 341. Such an architecture, although performing very well and adapted to certain applications, may have some drawbacks. Among others, gear pumps, even of small size, are not well adapted to generating small flow rates of medium pressure, such as those which are required in the continuous jet technique. This type of pump, by its construction, has internal leaks due to the necessary functional mechanical clearances: These leaks are such that the pump, in order to operate under good conditions, must generate a real flow rate very appreciably greater than that which is required for the jet. High flow rates at the pressures required involve mechanical and electric powers without any common measure with what is required for the jet, and so overheating, over dimensioned ventilation and electric supply.
Furthermore, the reliability of this type of pump, for this application, is very modest because materials compatible with light solvents (such as methyl ethyl ketone, are rare. The gears are often made from Teflon) which material has limited mechanical wear characteristics.
For the correct operation of such a circuit, it is necessary to use multiple sensors, such as pressure sensors, level sensors with immersed probes, viscosimeters, temperature sensors, for correcting the viscosity of the ink, considerable piping, etc. In addition, the cleaning procedures are tedious.
In another type of equipment, compressed air is used for pressurization. If it is industrial compressed air, it must be carefully filtered. The depression function for recovering the jet is obtained by a venturi effect. The major drawback of this supply system is the transfer of ink from the depressurized part to the pressurized part, which requires the provision of multiple transfer air locks. Furthermore, if compressed air is not available, a compressor is necessary.